1. Field of the Invention
The present invention relates to power supply circuits, and more particularly to voltage references in electronic circuits.
2. Description of Related Art
Many electronic circuits rely on a constant, reliable voltage source for providing power to various elements within the circuit. Although many electronic circuits receive a V.sub.CC voltage power supply and a V.sub.SS ground reference voltage from an external power supply, voltage reference signals may not be entirely constant with respect to time. Spurious surges, noise and voltage fluctuations may arise due to any number of sources, including other devices in the circuit transitioning state or developing performance problems.
A bandgap voltage reference addresses the problem of fluctuating reference voltages. Bandgap voltage references provide a substantially constant and reliable output voltage controlled by transistors and resistors within a voltage reference circuit. Bandage voltage reference circuits provide an output signal having a voltage that depends primarily on the ratio of various resistive elements and on the numbers and the arrangement of transistors, with only a slight dependency on the stability of the external power supply.
Voltage reference circuits are typically configured as "delta V.sub.BE " style generators. The output voltage of the voltage reference circuit is determined by a set of two equations having two independent simultaneous solutions. The first solution defines an active state in which the voltage reference circuit provides its reference voltage with a substantially constant power supply voltage. The second solution defines an inactive state in which the voltage reference circuit output (V.sub.BG) is approximately equal to the V.sub.SS (i.e., the ground voltage).
Bandgap voltage reference circuits have typically provided an output signal whenever the external voltages (i.e., V.sub.CC and V.sub.SS) are applied. Upon startup, however, such circuits may start in the inactive state.
To avoid initializing in the inactive state, many bandgap reference generators have included a startup circuit coupled to the bandgap voltage reference. The startup circuit receives an indication of whether an internal node within the bandgap voltage reference circuit has a voltage corresponding to the inactive state, and responds to the reference circuit being in that inactive state by driving the reference circuit to the active state.
Although voltage reference circuits are useful in many situations, it has not always proven advantageous for the voltage reference to be on. For example, many portable computers require a low power consumption mode, such as a sleep mode, while in other electronic circuitry several different power supply circuits may be included. In another words, it is at times advantageous for the voltage reference to be switched into the inactive state in which the output voltage, V.sub.BG, is zero.
Thus, although the startup circuit is useful for driving the voltage reference circuit output to the active state during normal operation, the presence of a startup circuit may be disadvantageous when it is desired for the voltage reference circuits that provide a grounded output.